Photographic recording of sound



G. L. DIMMICK PHOTOGRAPHIC RECORDING OF SOUND Mar ch 16, 1937.

Filed May- 9, 1932 3 A 7 5 W P M 4 R fir M R 4/ F. m M A INVENTOR e.|DIMMICK 1 BY g Q ATTORNEY Patented Mar. 16, 1937 UNITED STATES PATENTOFFICE i Glenn L. Dimmick, Westmont, N. J., assignor to RadioCorporation of America, a corporation of Delaware Application May 9,1932, Serial No. 610,301

3 Claims.

This invention relates to the photographic recording of sound and hasfor its principal object the provision of an apparatus and method ofoperation whereby the effect of decrease in the resolving power of therecord material as the frequency of the recorded sound increases isobviated.

'A further object is the provision of means for overcoming thedistortion resulting from de "1 6 crease in the resolving power of therecord mate-' rial with increase in frequency of the recorded sound byartificially producing a similar distortion of opposite phase.

A further object is the provision of a sound "it recorder wherein-theamplified audio impulses are utilized to actuate the recording elementand an auxiliary circuit responsive to the high frequency components ofthese impulses is arranged to modify the operation of the recordingelement in a manner to neutralize variation in the resolving power ofthe record material.

It is well known that objectionable noise, commonly known as groundnoise, is produced by scratches and dust on the clear part of a soundrecord. Various means have been provided for eliminating suchdistortion. In the variable area type of recording they assume the formof a shutter which is caused to follow the envelope of the sound recordand to reduce the clear portion of the record to the minimum requiredfor proper reproduction of sound. In the variable density type ofrecording they assume the form of a biased light valve. Neither of theseforms, however, have satisfactorily solved the problem of eliminatingthe undesirable effects of decrease in the resolving power of the recordmaterial as the frequency of the recorded sound increases. This effectis'particularly disagreeable in the case of small records such as are 40used in connection with 16 millimeter recordings. In accordance with thepresent invention, it is avoided by the provision of means forsubjecting the recording element to a biasing force which is dependenton the high frequency components of the recorded sound.

The invention will be better understood from the following descriptionwhen considered in connection with the accompanying drawing and itsscope will be pointed out in the appended claims.

Referring to the drawing,

Figs. 1 to 3 are explanatory diagrams, and

Fig. 4 is a diagrammatic showing of a sound recorder wherein theinvention has been embodied.

Fig. 1 illustrates a sound track including an unmodulated section A anda fully modulated constant frequency section B. If the exposed portionof the unmodulated section A is perfectly black it is evident that theaverage transmission 5 for this section would be fifty percent. Also, ifthe sine wave modulation of the constant frequency section B isperfectly recorded the average transmission or resolving power is stillfifty percent. It has been found, however, that the 10 resolving powerof the photographic film ordinarily utilized in sound recording is notperfect but decreases with increase in the frequency of the recordedsound.

As shown by Fig. 2 the valleys of the Waves 5 are badly fogged, thedensity being greatest near the base and least near the top. It is quiteapparent that the average transmission of a high frequency sound tracksuch as that illustrated is less than if the track were unmodulated. Itis also apparent that the light modulation produced by the track shownin Fig. 2 is necessarily less than that produced by section B of thesound track shown by Fig. 1, and that the peaks contribute more to thelight modulation than do the valleys.

When a sibilant sound is recorded on the film the high frequencies varyin amplitude. Such a variation is represented in Fig. 3. Fogging of thevalleys in this case gives rise to a new wave not present in theoriginal sound. This new wave is approximately the same shape as thecurve of the envelope of the high frequencies and is represented in Fig.3 by a dotted line. When the sibilant sound covering the pronunciationof the letter S is reproduced from a 16 millimeter film a lowerfrequency for noise is usually held. This noise is proportional to theintensity of the sibilant sound and especially pronounced in records inwhich the high frequencies have been boosted to compensate for the lossdue to the film.

As will be apparent to those skilled in the art, the invention isapplicable to different types of recording apparatus. In order tosimplify 45 an understanding of it, it is illustrated as utilized inconnection with a galvanometer type of recorder.

Referring more particularly to Fig. l, the apparatus includes amicrophone ill, a microphone 5 amplifier I l which may be mounted in thesame casing as the microphone, a mixer I2, an amplifier l3, agalvanometer recorder M, and an auxiliary high frequency responsivecircuit Hi.

It will be apparent that the recorder is of the 55 usual type exceptthat it is provided with two actuating coils l6 and Il. It includes alight source I8, a lens [9, a light stop 20, a lens 2|, a mirror 22which is actuated by the coils l6 and il, a gauge 23, a cylindrical lens24, a spherical lens 25, a slit 26 and an objective 21.

The output circuit of the amplifier I3 is connected directly to thegalvanometer coil 16 and is connected through the auxiliary circuit l tothe galvanometer coil H. The auxiliary circuit l5 includes a transformerprovided with a winding 28 and a winding 29 which is connected to theoutput of the amplifier I3 and is shunted by a filter 30-3! which allowsmore high frequencies than low frequencies to pass from the amplifier 3to the transformer 2329. The opposite terminals of the secondary winding28 are connected to the grids 32 and 33 of a pair of half waverectifiers 34 and 35. It will be observed that the anodes 36 and 31 ofthese rectifiers are connected together and that their cathodes 38 and39 are also connected together. A battery 40 is provided for supplyingheating current to the cathodes. The anodes 36 and 31 are connected toone terminal of the galvanometer coil ll through inductance M, and theother terminal of the coil I1 is connected to the cathodes 38 and 39through a battery 42. A capacity 43 may be connected across thegalvanometer coil H and the inductance Al for the purpose of smoothingout the rectified current, although this filtering action is not alwaysnecessary for satisfactory operation of the apparatus. It will beunderstood that a half wave rectifier may be substituted for the fullwave rectifier, as found expedient.

The operation of the apparatus will be readily understood. Thegalvanometer coil I 6 functions in a Well known manner to vibrate themirror 22 and vary the Width of the sound track recorded on the film.The galvanometer coil ll responds only to the higher frequencycomponents of the recorded sound and functions to shift the meanposition, or change the zero point, of the recording light beam by anamount equal to the change in the resolving power of the film at theparticular frequency being recorded. The mean transmission of the filmis thus made constant for all frequencies in the useful range anddistortion and spurious frequencies as described above, due to change inthe resolving power of the film with frequency, are eliminated.

Having thus described my invention, What I claim is:

'1. The combination of a source of electrical impulses, a light source,means for directing light from said source onto a photographic film,means in the path of said light for modulating the light in accordancewith said impulses, means connected to said source to isolate highfrequency impulses therefrom, and means connected to said isolatingmeans for shifting the zero point of said modulating means in accordancewith the envelope of said isolated impulses, whereby spurious envelopefrequencies are neutralized.

2. The combination of a source of electrical impulses, a light source,means for directing light from said source onto a photographic film,means in the path of said light for modulating the light in accordancewith said impulses, means connected to said source to isolate highfrequency impulses therefrom, and means connecting said isolating meansto said modulating means for shifting the zero point of said modulatingmeans in accordance with the envelope of said isolated impulses, wherebyspurious envelope frequencies are neutralized.

3. The combination of a source of electrical impulses, a light source,means for focussing a narrow line of light from said source onto aphotographic film, means in the path of said light for modulating thelight in accordance with said impulses, means connected to said sourceto isolate high frequency impulses therefrom, and means connecting saidisolating means to said modulating means for shifting the zero point ofsaid modulating means in accordance with the envelope of said isolatedimpulses, whereby spurious envelope frequencies are neutralized.

GLENN L. DIMMICK.

